1. Field of the Invention
This invention relates to an improved process for the production of maleic anhydride by the catalytic oxidation of n-butane.
2. Description of the Related Art
It is well known in the art that maleic anhydride may be manufactured by the vapor phase oxidation of n-butane as it flows through a fixed bed reactor containing a vanadium phosphorus oxide (VPO) catalyst. N-butane in admixture with air is brought into contact with a VPO catalyst under conditions such that the n-butane is oxidized to maleic anhydride. The effluent from the reactor may be cooled to partially condense the product maleic anhydride from the effluent gases. The gaseous product, with or without partial maleic anhydride removal, is scrubbed using a solvent, usually water or an organic solvent, to recover the maleic anhydride. The remaining gases, containing unconverted n-butane, are commonly incinerated in an effluent gas incinerator prior to venting to atmosphere. Useful processes, as well as VPO catalysts for the production of maleic anhydride from n-butane are known from U.S. Pat. Nos. 6,194,587; 5,360,916 and 6,858,561, which are incorporated herein by reference. U.S. Pat. No. 5,360,916 discloses a two stage process for maleic anhydride production wherein n-butane is oxidized in a first reaction zone and the effluent from this zone is passed together with supplemental n-butane to a second reactor in series in order to complete production of the maleic anhydride. In U.S. Pat. No. 5,360,916, butane is oxidized with molecular oxygen in a dilute state by bringing a mixture of vaporized butane and air having a controlled butane content into the presence of a vanadium phosphorus oxide catalyst in a first oxidation zone under controlled pressure and temperature conditions, cooling the gaseous effluent from the first oxidation zone to a temperature in the range of 50° C. to 300° C., introducing a controlled amount of butane into the cooled gaseous effluent from the first oxidation zone, introducing the cooled butane-enriched stream into the second oxidation zone, and bringing the thus butane-enriched mixture into contact with a vanadium-phosphorus-oxygen catalyst disposed in the second zone. U.S. Pat. No. 6,858,561 teaches a process for preparing a catalyst for maleic anhydride production.
During the known processes for the production of maleic anhydride by the catalytic oxidation of n-butane, acrylic acid is formed as a nuisance by-product of the reaction. In maleic anhydride plants, downstream process equipment such as heat exchangers, and distillation equipment are often fouled after prolonged operation with a polyacrylic polymer derived from the accumulation and self-reaction of acrylic acid. Such fouling occurs where the acrylic acid polymerizes and obstructs and reduces the performance of the distillation column. There is therefore a need in the art for an improved process which inhibits the formation of acrylic acid by-product while preserving maleic anhydride yield.
One method for reducing acrylic acid production suggested by U.S. Pat. No. 5,945,368 is to employ a phosphorus vanadium oxide catalyst containing molybdenum. While this technique apparently reduces the amount of acrylic acid produced, the yield of maleic anhydride is also reduced.
In a typical commercial process for the production of maleic anhydride by the catalytic oxidation of n-butane, the rate of n-butane conversion to maleic anhydride is approximately 82 wt. %-83 wt. % which gives a maleic anhydride yield of 97+wt. % and an acrylic acid yield of approximately 1.7 wt. % to 1.8 wt. %. Under the conditions of the present improvement, conversion of n-butane to maleic anhydride is increased to about 84% or more, and acrylic acid yield is reduced, typically to below 1.4 wt %, without decreasing maleic anhydride yield.